Aramid polymers have been known for decades, but improvement of some of their properties is still an object. It was the objective of the present invention to provide aramid derivatives and fibers thereof having high compressive properties, high tenacity, and high breaking force.
Several attempts to improve compressive properties of polymeric fibers in general, and aramid fibers in particular, are known. Sweeny reported the thermal elimination of activated aryl halogens followed by the combination of the aryl free-moieties, in order to create inter-chain covalent bonding (Sweeny, W., Improvements in compressive properties of high modulus fibers by crosslinking. J. Polym. Sci., Part A: Polym. Chem., 1992, 30(6): p. 1111-1122). The increase in compressive strength was accompanied by a significant drop in fiber toughness. Markoski (Markoski, L. J., et al., Cross-linkable copolymers of poly(p-phenyleneterephthalamide); Chem. Mat., 1993, 5(3): p. 248-250) investigated the (partial) substitution of terephthalic acid by cross-linkable benzocyclobutene containing terephthalic acid in poly(p-phenylene-terephthalamide). Other approaches based on several types of post treatments were reported by Sweeney (Sweeney, D. J., et al., Influence of Thermal Treatment Conditions on the Recoil Compressive Strength of Kevlar-29 fibers. High Perform. Polym., 2002, 14(2): p. 133-143) who investigated the influence of heat treatments on the compressive properties of aramid fibers, however at the cost of the tensile strengths.
In JP 11172012 crosslinking of an aramid silicone polymer was described, but this process was performed using a mixed polymer, not typically an aramid polymer. This material was suitable for making contact lenses, and no compressive strength improvement was discovered or disclosed.
In WO 2008/028605 crosslinking of aramids was described wherein the aromatic moieties of the aramid were modified to obtain cross-linkable copolymers by substitution with carboxylic acid and hydroxy groups. These aramids were crosslinked through their aromatic hydroxy and/or carboxylic acid groups.